This invention relates to the field of particle accelerators or, more particularly, to the field of ion accelerators. These devices are used in nuclear physics, in various methods of analysis in geological prospecting, in geophysics, and in radiation chemistry.
Ion accelerators are well-known tools for ion bombardment used in physics and technology. The use of such devices in industry places on them the demands of portability and transportability. Devices developed in accordance with these demands contain an ion source, accelerating electrodes, and a target unit.
In order to make the device operative it is subjected to vacuum treatment. After the device has been unsoldered from the vacuum installation where evacuation was performed from its cavity, the pressure inside the device does not stay constant, for internal surfaces of the device give off residual gases which are impossible to remove completely in the course of vacuum treatment. Furthermore, a portion of the working gas (i.e. the gas whose ions are accelerated in the device) is absorbed in the target and other components of the accelerator.
Included in known devices, therefore, is a pressure control unit. It is constructed as a conductor which is placed in the accelerator cavity and has leads passing to the outside. In the course of vacuum treatment a portion of the conductor placed in the accelerator cavity is saturated with gas. As required, the gas is expelled from the conductor by heating the latter with electric current. The absorption of the liberated gas takes place during conductor cooling after the electric current has been switched off. In practice, however, the gas evolved in the course of heating is not absorbed completely, so that gas pressure control can be effected within a limited range of magnituide. It is impossible to control gas phase composition inside the device.
It should be added that in the course of evacuation particles of oils and other materials penetrate inside the device from the pumps and faucets of the vacuum installation to be subsequently accumulated on the target surface. Upon accleration, the ions of foreign gases reach the target and in doing so form a parasitic constituent of the ion current to the target. All this results in lowered accelerator efficiency.
Foreign gases penetrating the cavity of the device bring about changes in ionization conditions, i.e. reduce the total amount of ions of the working (ionized) gas by lowering the amount of atomic ions most effective in target bombardment.
As a result, certain characteristics of unsoldered accelerators are substantially lower than those for stationary devices. For example, under optimal conditions realizable in stationary deuton accelerators which bombard tritium targets for neutron production, up to 10.sup.8 neutron/sec are produced at 150 kV and a 1 MA current, whereas in portable unsoldered accelerators the rate is by almost two orders of magnitude below this figure constituting about 10.sup.6 neutrons/sec at the same feed parameters.
The above traces of vacuum oil and cement vapors are sorbed on the surfaces of accelerator components, these materials decomposing on the target under the influence of ion bombardment. Thus the cracking process takes place. Heavy hydrocarbons and carbon formed can no longer desorb from the surface, which leads to the development of an inert layer ever increasing in the course of time, and in this layer the accelerated ions bombarding the target lose their energy with no other effect than heating the target. In stationary accelerators the contaminated target is replaced, and the service life of the device is practically limitless. In an unsoldered accelerator, target contamination and the reduction of tritium concentration which take place in the course of operation limit the service life to 100-150 hours.
And, lastly, the dimensions of the ion source of a portable accelertor are limited, and this results in somewhat over-high pressure under which the source operates, since otherwise a sufficient quantity of ions cannot be obtained in a given volume. The increase in pressure, however, reduces the percentage of the more effective atomic ions and impairs conditions for ion acceleration in the acceleration gas where the pressure is the same as in the ion source.
In order to overcome the ageing of the target and to preclude the failure of the entire device, an arrangement for renovating the target surface layer and, in addition, an arrangement for regulation of gas composition and pressure control have been incorporated in its design. Thus, the invention discussed herein has for its objects
an arrangement for regulation of gas composition and pressure control;
improvement of the ion source design;
an arrangement for target renovation;
the use in the arrangement of such material as will be sufficient and necessary for the successful operation of the proposed ion accelerator.
Another object of this invention is a method for increasing the efficiency of operation of the proposed ion accelerator. The above objects of the invention will be understood from the following description and the explanation by means of the appended drawings.